i) Field of the Invention
The present invention relates to a catalyst, a membrane electrode assembly, a fuel cell, and a method for producing the catalyst.
ii) Related Art
Recently, a fuel cell has been studied to be put to practical use for a vehicle, a home, or the like. For example, a polymer electrolyte fuel cell using a methanol solution as fuel can be reduced in size and weight and can be continuously driven by supplying fuel. Accordingly, the fuel cell is actively studied currently. However, the performance thereof does not reach a level to come into wide use. One of the reasons is the insufficient activity of an electrode catalyst. Since the fuel cell converts chemical energy into electric energy by catalysis reaction in the electrode, a catalyst with a high activity is indispensable to develop a high-performance fuel cell. Generally, PtRu is used as an anode catalyst of the fuel cell, but a voltage loss by the PtRu catalyst is about 0.3 V in a theoretical voltage 1.21 V which can be obtained by the catalysis reaction in the electrode. In addition, when electricity is generated for a long time, the catalyst is deteriorated to result in decreasing the performance of the fuel cell. Thus, there has been required an anode catalyst which is superior to PtRu and is stabilized in spite of the generation of electricity for a long time.
The study on the catalyst has been conducted in many fields. In U.S. Pat. No. 3,506,494, a catalyst using a ternary alloy of Pt, Ru, and a third metal (Au, Re, Ta, W, Mo, Rh, etc.) as an electrode catalyst is disclosed. In JP-A-2005-259557 (KOKAI), there is disclosed a catalyst in which as catalyst metal components, platinum, ruthenium, and at least one selected from an element of Group 4, an element of Group 5, and an element of Group 6 in the periodic table are supported by a conductive carrier. In JP-A-2006-224095 (KOKAI), there is disclosed a catalyst in which at least one element selected from Pt, Ru, Pd, and the like; and at least one element of Fe, Mo, W, and the like are supported as a catalyst. All of the above catalysts are produced by a solution method.
The above-mentioned solution method has been generally used for catalyst composition. However, the solution method has an issue that it is difficult to perform a structure control and a surface control of a catalyst for an element which is scarcely reduced and an element which scarcely alloys. Although a catalyst synthesis using a sputtering method and a vapor deposition method has an advantage for a material control, a study on an influence of a process such as an element kind, a catalyst constitution, a substrate material, and a substrate temperature has been little conducted yet. Since not a few of catalyst particles are nano-particles, a surface electron state of the catalyst particles and a nano-structure of the catalyst particles tend to greatly depend on a kind and the amount of elements added to the catalyst particles. In order to obtain catalyst particles with a high activity and a high stability, it is required to appropriately adjust the kind of the elements added to the catalyst particles, the amount of the elements, and the combination of the elements.
In U.S. Pat. No. 6,171,721, a catalyst synthesized by a sputtering method is disclosed. However, a study on a composition of elements added to catalyst particles is not sufficient yet, and a sufficient methanol oxidation activity and stability are not established.